A method of the above described general type is disclosed in European Patent 0 473 569 B1. In the known method, digital data are exchanged between a base station and a passive transponder by means of an amplitude modulated carrier wave. The individual bits of a data word consist of pulses and gaps in the electromagnetic field, i.e. a time interval in which the electromagnetic field is switched on and a time interval in which the electromagnetic field is switched off (field gap). The field gap serves as a separator between two successive bits. The respective significant value of the bits is determined by the length or duration of the time interval during which the electromagnetic field is switched on for defining the respective particular bit. Furthermore, in the passive system of the known arrangement, the energy needed for operating the transponder is generated or extracted from the carrier field by means of absorption modulation.
A further method pertinent to the present general field is known from the German Patent Laying-Open document DE 100 50 878 A1. The disclosed method uses various different modulation indices for the amplitude modulation of the carrier wave, whereby the data transmission rate is increased, in that several information symbols are transmitted using a single carrier wave. For this purpose, the carrier wave is modulated by means of a prescribed allocation or correlation between the information symbols and the modulation indices. It is a disadvantage that the separation of the information symbols requires a sensitive demodulator circuit, which limits the reach or range of the communication under unfavorable reception conditions.
In general, methods for transmitting data between a base station and a transponder are used in order to carry out an identification or verification in a so-called authentication process. In order that the user of the system does not notice any delay, this process must be carried out and completed within a time span of approximately 100 ms. In view of the ever increasing security requirements, a great number of information packets must be modulated on a carrier wave in ever shorter time spans. The modulation of the information packets may be carried out using amplitude modulation (for example in the manner of amplitude shift keying: ASK) or especially phase modulation (e.g. phase shift keying: PSK) and frequency modulation (e.g. frequency shift keying: FSK) in connection with a pulse width modulation (PWM).
In order to achieve a higher data transmission rate and a greater transmission range, carrier frequencies in the range of ultra high frequencies (UHF) and microwaves are being utilized more often in connection with transponders, whereby the various different national high frequency (HF) transmission regulations influence the data transmission rate through the prescribed frequency bands and/or the width of the side bands.
In any event, a data protocol forms the basis of bi-directional data transmission between a transponder and a base station, whereby this data protocol comprises a header section, a middle section, and an end section. In the header section, among other things, the number of the information symbols and the number of the available significant values of the data bits are specified or defined by a characteristic identifier or identification for the individual symbols. In the middle section of the data protocol, the data are then encoded using the specified identification of the symbols. Such a scheme is, for example, proposed in the International Standards organization (ISO) Working Draft Proposal ISO 18000-6 of February 2001. In order to achieve a reliable data transmission, both any unfavorable communication conditions as well as varying switch-on and switch-off transient characteristics of the transmitting and receiving units of the transponder and of the base station respectively are taken into account. This is achieved by allocating to the symbols, as the characteristic identification thereof, respective modulation intervals having an especially long time duration. Furthermore, in connection with frequencies in the UHF or microwave range, the time duration or length of the modulation intervals is increased, in order to thereby account for the inaccuracy in the determination of the modulation intervals by means of the free-running oscillators or RC charging curves, in other words, for example, in order to avoid read errors in the data transmission.
A disadvantage of the previously known methods is that the relatively long time duration of the modulation intervals necessarily reduces the data transmission rate. The data transmission rate is further reduced in that the modulation stroke or range is adapted to the narrowest permissible side band, in order to thereby satisfy the various national HF regulations or requirements using a single protocol or protocol adjustment. This leads to constantly low data transmission rates. Such low data transmission rates become especially noticeable and problematic as an interfering influence in various time-critical applications.